Sadly, due to Sonos’ odd naming choices, some of your post is raising a question. Not, I suspect, your fault, but I need to ask about:

1. You say playbar and play bar. Do you mean the Sonos PLAYBAR, or merely a Sonos soundbar of some type?
2. in your second ‘room’, you indicate four ‘rear’ speakers. Do you intend to muddle the surround affect by multiple points of sound?

In answer to your specific questions:

Sonos doesn’t make any in wall speakers, but they do sell speakers from a company called Sonance. 

Yes, in order to play one pair of speakers in a wc (I assume this is a water closet, more commonly known in the US as a bathroom), you would need three separate Sonos Amps. 

I don’t know what ‘surface mount mains ones’ means. 

Not sure how an Audio Switch would help in the setups listed.

If you want different music simultaneously playing in each of the wc’s you’ll need a separate AMP for each room. You could install an in-wall speaker Volume control in each wc to adjust the Volume separately in each wc and connect the controls to the same AMP..

Hi 

This is the room layout. In regard to 4 speakers. Is the way to go? I’ve attached an image of the large area and small room. Green being the PLAYBAR, the red and black are indicative positions of speakers. The smaller of the two rooms will be more of a cinema room with one of my older Sonos subs. The larger area is less acoustically friendly and for general tv and music when entertaining
 

by “mains” I mean 230v speakers recessed, Sonos seem to do a large range of speakers but not for this type which will now prove costly due to the fact I will need an amp for each bathroom if I don’t use a volume control in each space. In my previous house I just installed a play 1 connected to mains and job was a gooden. I’m trying to justify the cost to myself. 

Surround sound works best when the listener is within the front-rear speaker layout. In the larger room with four ceiling surrounds you won’t have this optimal arrangement. I suggest that you install Volume controls on each pair of the ceiling speakers. For TV viewing (surround) you can mute the ‘other’ pair. When listening to music you can use all speakers and adjust each to suit the occasion.

Thank you. That’s sounds like a good idea. Would it just be a case of wiring both sets in parallel (a pair in each terminal of amp) and the furthest set in series with a volume controller?

do you think 6 inch Sonos would suffice or would you go with the 8?

finally, if I link the playbar to the tv with an optical cable and the Sonos amp network cabled, would i incur any sound delay sync issues when watching tv?

Each speaker would be wired to a control and the controls would be wired in parallel at the AMP. Use a control on all speakers.

A soundbar will minimize lip sync issues while watching TV. The AMP would be configured as the soundbar’s surround amplifier. As far as SONOS is concerned the soundbar and AMP would be part of the same Room.

I’d prefer the larger speakers.

Thank you,

do you recommendation for a volume controller? It would be good if it was app based to change channels via tablet etc. 

Russound ALT-125R is a good Volume control choice.

In my opinion, adding another control system simply to deal with that second pair of ceiling speakers would add extra cost and unnecessary operational complexity. But, if you are a tinkerer … there are lots of tinker options for DIY’s.